1. Field of the Invention
The present invention relates to a zinciferous coated steel sheet and a method for producing the same.
2. Description of the Related Arts
Owing to various advantages, zinciferous coated steel sheets are widely used as rust-proof steel sheets. For utilizing the zinciferous coated steel sheets as the rust-proof steel sheets for automobiles, however, excellent press-formability and adhesiveness are requested as the characteristics requirements in the car body manufacturing line, as well as corrosion resistance and the like.
Zinciferous coated steel sheets, however, generally have a disadvantage of inferiority in press-formability to cold-rolled steel sheets. The drawback results from a large sliding resistance between the zinciferous coated steel sheet and the press mold compared with that observed in cold-rolled steel sheets. That is, the large sliding resistance interferes with the entering of the zinciferous coated steel sheet into the press mold at a portion where vigorous sliding occurs between the bead and the zinciferous coated steel sheet, which tends to induce fracture of the steel sheet.
There is a common practice of applying high viscosity lubricant to improve the press-formability of zinciferous coated steel sheet. This method has, however, problems that the viscous lubricant induces coating defects during the coating process caused by insufficient degreasing, and that lack of oil during the pressing stage results in unstable press performance. Therefore, improvement of press-formability of zinciferous coated steel sheets is strongly desired.
In addition, in the manufacturing line of automobile bodies, various kinds of adhesives are used for anti-rusting and damping of car bodies. In recent years, it was found that the adhesiveness of zinciferous coated steel sheets is inferior to that of cold-rolled steel sheets. Accordingly, improvement of adhesiveness of zinciferous coated steel sheets is also desired.
As a measure to solve the above-described problems, Japanese Patent Laid-Open No. 53-60332 and No. 2-190483 disclose technology to form an oxide film consisting mainly of ZnO on the surface of zinciferous coated steel sheet through electrolysis treatment, immersion treatment, applying-oxidizing treatment, or heating treatment: (hereinafter the technology is referred to as "Prior Art 1").
Japanese Patent Laid-Open No. 4-88196 discloses technology to improve press-formability and chemical treatability by forming an oxide film consisting mainly of P-oxide on the surface of zinciferous coated steel sheet by immersing the coating steel sheet in an aqueous solution containing 5 to 60 g/liter of sodium phosphate, or by electrolysis treatment, or by spraying the above-described aqueous solution: (hereinafter the technology is referred to as "Prior Art 2").
Japanese Patent Laid-Open No. 3-191093 discloses technology to improve press-formability and chemical treatability by forming a Ni-oxide on the surface of zinciferous coated steel sheet through electrolysis treatment, immersion treatment, applying treatment, applying-oxidizing treatment, or heating treatment: (hereinafter the technology referred to as "Prior Art 3").
Japanese Patent Laid-Open No. 58-67885 discloses technology to improve corrosion resistance by forming a film of metal such as Ni and Fe on the surface of zinciferous coated steel sheet through the film-forming method is not particularly specified: (hereinafter the technology is referred to as "Prior Art 4").
The above-described Prior Arts have drawbacks described below.
Since Prior Art 1 is a method to form an oxide consisting mainly of ZnO on the surface of the coating layer, the workability is improved, but there appears less of improvement of press-formability because the sliding resistance between the press mold and the coated steel sheet does not sufficiently reduce. In addition, it has been identified that, if an oxide consisting mainly of ZnO exists on the surface of the steel sheet, the adhesiveness is further degraded.
Since Prior Art 2 is a method to form an oxide film consisting mainly of P-oxide on the surface of zinciferous coated steel sheet, it has a problem of degrading the adhesiveness, though the effect of improvement of press-formability and chemical treatability is high.
Since Prior Art 3 is a method to form a film of Ni-oxide single phase, it has not sufficient effect to improve adhesiveness, though the press-formability is improved.
Since Prior Art 4 is a method to form a film of metal such as Ni, it cannot give satisfactory adhesiveness owing to poor wettability against adhesives because of strong metallic characteristics of the film, though the corrosion resistance is improved.